Your child loved the idea of Percy Jackson — gods loose in the ordinary world, an ordinary child at the center of it — and wants more like it. This is the gentler end of that shelf. If Percy itself was a lot (375 pages, real deaths, a menacing Underworld, monster-fighting with no quiet chapters, and a hero whose father is gone before page one and whose mother is killed in front of him in chapter one), some of these six ease off on one or more of those counts: shorter, less frightening, or with a family that stays whole. A couple sit right at Percy's intensity and are here for the child who wants more myth, not less — they open a mythology no one at school has heard of. Every note says which is which. If your child is ready to climb instead of ease off, the trail After Percy Jackson: More Myth is the step up.
The top two are the genuinely gentle ones. The Unicorn Rescue Society is a third of Percy's length with a drawing every few pages, the creatures get rescued instead of killed, and Elliot's parents are alive and unremarkably present, which sounds like nothing and is exactly the thing some Percy-anxious children need. Goddess Girls is the Olympians as middle schoolers, where the worst thing is a snooty girl named Medusa. The Sisters Grimm and The Land of Stories sit in the middle: adventure-scary rather than frightening, no Underworld, but both built on missing or dead parents, so they take the fear down and leave the family question. Then the last two. The Serpent's Secret and Aru Shah are the best books here, and they are not gentler than Percy — same length, same stakes, same absent god parent, each opening by taking a child's parents away. They're on this shelf for the Percy fan who wants a new mythology, not a calmer read. If length or on-page death was the trouble, skip them. If it wasn't, they're wonderful.